The elongation of thin and thick filaments during isometric contraction of a molluscan smooth muscle was studied by measuring spacing changes of meridional reflections in the medium-angle X-ray diffraction pattern. X-ray patterns from the anterior byssus retractor muscle of Mytilus edulis in the resting, active, and catch states were taken from the same part of a muscle bundle at a fixed specimen-to-detector distance, using imaging plates and 10 s exposure to synchrotron radiation. The third-order reflection (9.2 A) of the axial period of actin, and the fourteenth-order reflection (10.4 A) of the axial subunit-repeat of the thick filament are increased in spacing in the active and catch states. From accurately measured changes in the axial distance of the 9.2 A layer line from the origin, thin filament elongations in the active and catch states are estimated to be 0.48 and 0.32%, respectively, in a muscle that maintains a tension of 12.2 kg cm-2 in the active state and 9.8 kg cm-2 in the catch state. Thick filament elongations in the active and catch states are similarly estimated to be 0.33, and 0.28%, respectively, based on the axial shift of the 10.4 A reflection. The 0.48% elongation of the thin filament in the active state agrees with an elongation that is presumed by White and Thorson (1973) to estimate the lower limit of the thin-filament stiffness. It seems that in the catch state the activated and resting thin filament structures are intermixed. The activated parts of the thin filament are probably more elongated than the apparent value, 0.32%.

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Small-angle X-ray diffraction studies of the living anterior byssus retractor muscle (ABRM) of Mytilus edulis have been carried out using a long (2.2 m) point-focusing camera to investigate the overlap between the thin-and thick-®lament re¯ections. While the re¯ections due to the cross-bridge lattice on the thick ®laments in the resting ABRM are distant in the axial direction from the thin®lament layer lines, the paramyosin cores of the thick ®laments produce re¯ections very close to the thin-®lament re¯ections at axial spacings of 387, 59 and 51 A Ê . Using such a long camera it becomes possible to separate these re¯ections due to an improvement in the angular resolution and increase in the difference between the widths of the thin-and thick-®lament re¯ections, and it is shown that the intensities of the 387 and 59 A Ê thin-®lament layer lines gradually drop to zero close to the meridian. It becomes relatively easy to observe the thin-®lament-based X-ray pattern by use of the long camera, making it possible to show a partial resemblance of the small-angle X-ray pattern from the contracting ABRM with the pattern from the rigorized ABRM.

The spectral dependence of the F-center formation efficiency in KC1 and KBr was measured in the photon energy region of the CI K and Br K edges at liquid-helium temperature. No discernible increase in the F-center formation efficiency was observed at either the CI or Br K edge.